This invention generally relates to solid-state imaging devices and, more particularly, to an active pixel sensor column amplifier architecture in which fixed panel noise and other noise sources can be removed from an image signal generated by a set of pixels on the solid-state imaging device.
Past electronic cameras used charge coupled device (CCD) imagers to convert optical images into corresponding electronic signals. A typical CCD imager includes a matrix of photosites (an array) that are capable of collecting free electrons that form an electrical charge packet that is directly proportional to the photon radiation incident at that photosite. Accordingly, when an image is focused on the surface of the array, the charge packet at each photosite of the array corresponds to a respective picture element or pixel of the complete image. The generated photosite charge packets are transferred in a parallel manner into a CCD shift register where they are transferred serially as an output signal of the CCD image. The CCD imager output signal is a series of electronic signals whose amplitude represent the light intensity of the image at each pixel for a single image frame. The process is continuously repeated to generate output signals, which correspond to several image frames per second. Consequently, each frame in the generated output signal contains information concerning the detected light intensity at each photosite.
A drawback of typical CCD imagers is that they require near-perfect charge transfer over a distance of approximately 1-2 centimeters through semiconductor materials. Therefore, such CCD imagers require a substantial amount of power, are difficult to manufacture in large array sizes, and are difficult to integrate with complementary metal oxide semiconductor (CMOS) on chip electronics. The difficulty is that the standard CCD process is incompatible with CMOS processing, while the imager signal processing electronics required for the imager are best fabricated in CMOS.
Therefore, active pixel sensors (APS) have become increasingly popular as an alternative to CCD imagers in camera equipment. APS employ one or more on-chip transistors at each pixel photosite in the array. The transistors at the photosite buffer a photo signal corresponding to the charge packet generated by the photosite and drive an output signal line avoiding the disadvantages of the CCD imagers that have to serially shift the data out. However, typical APS arrays still generate an output signal with each frame (representing a row of image data in the array) containing information representing the detected light intensity at each photosite.
The traditional method for storing and buffering the outputs (see U.S. Pat. No. 5,471,515) of a row of pixels in an APS array is to individually store the image signal (that is integrated over an exposure time) and a reset level on two capacitors. The two capacitors are connected between some reference (such as ground (GND)) and the gates of a pair of source follower amplifiers. The outputs of the two source followers then provide a quasi-correlated double sampled difference image signal proportional to the integrated light that can be further buffered, amplified or digitized. However, this traditional method has several problems.
One problem is that a non-uniform background image will result due to fixed pattern noise (FPN). This FPN noise appears as a random streaking appearing on a blank display. This effect is due to process induced mismatches because the pair of source follower amplifiers, while schematically identical, will have different offset voltages which result in a random offset (for each column) in the output difference signal. This noise is bad in that it is inherent in the design and manufacture of the APS array, thus varying from part to part. It cannot be removed without additional complicated circuitry that uses scarce integrated circuit real estate, thus increasing the cost of an integrated image sensor.
A second problem is that the resolution of the image from the APS array is not sharp enough as required by the increased consumer demands for quality vibrant images. This effect is due to the source follower amps having less than unity gain, which reduces the sensitivity, and thus the available signal to noise ratio (S/N) of the signal. Thus, any noise added after the voltage follower will appear larger relative to the actual image. This effect is especially noticeable in low light conditions. Most people who have used a video camera are familiar with this problem. Users do not want to use flashes or photo lamps due to their cost, power requirements, obnoxiousness, and general clumsiness of setup. User demands thus require the ability to get high quality photos in all lighting conditions, including low light situations. Therefore, a need to reduce the noise and/or increase the image signal on the APS array is needed.
A third problem sometimes noticed with certain images is that the picture appears distorted. This distortion is due to the non-linearity of the source follower amplifiers inherent in their design. Because there is no feedback in the source follower amplifier, the non-linearity cannot be corrected. The user demands that a camera always present a clear undistorted image to preserve accurately those treasured memories they wish to keep.
A less severe fourth problem is that some APS arrays present an image that has a snowy noise effect even in well-lit conditions. This noise condition can arise because the source follower amplifiers consume a lot of power and therefore are turned off when that particular column of the APS array is not being accessed. When the source follower amps are turned on to sample a column, extra charge from the switching circuit introduces this noise onto the signal. Another way this noise is introduced is due to common mode noise on each source follower amplifier (from the power supply, substrate, or other switching circuits) which is coupled onto the image signal. Because the common mode noise frequency is usually independent from the rate at which the image is being sampled from the APS array, the noise tends to appear random and thus snowy to the user. Again, the users are demanding defect free images and generally will not tolerate this noise.
Some implementations have tried to solve these noise problems by using a darkened column as a reference source of noise which is subtracted from the other photocells columns in the array that are integrating light. However any difference in leakage from the photocell to the substrate, caused by defects in the IC processing, between the reference column photosites and the image column photosites will be an additional noise source for fixed pattern noise (FPN), which the implementations are trying to eliminate.
Some implementations that use a passive pixel site (no buffering or gain transistor is used at photosite 10) use a complex operational transconductance amplifier in the column amplifier to alleviate the problems associated with the voltage follower amplifiers traditionally used. However, in addition to the complexity and large amount of IC real estate required, the gain of the amplifiers are inconsistent across the IC due to variations in the manufacturing process, once again adding a new source of FPN.
What is required to accelerate the market for digital photography is an innovative method of removing noise sources from the image signal without adding substantial cost or manufacturing difficulties, thus providing professional results for ordinary consumers.
A pixel column amplifier architecture creates a reduced noise differential image signal from an pixel sensor array. The pixel column amplifier architecture comprises a first double sampling (DS) circuit and a second DS circuit that has the same configuration as the first DS circuit. An image signal containing a combination of noise components created on a substrate is coupled to the first DS circuit. A reference image signal, held in a reset state, represents the noise component of the image signal and is coupled to the second DS circuit. Further, a reference voltage source is coupled to a reference input of both the first DS and the second DS circuits. The first DS circuit provides the first side of the differential image signal, and the second DS circuit provides the second side of the differential image signal.